This invention relates in general to optical devices and in particular to an adjustable objective holder in such devices like microfiche readers, overhead projectors and the like in which the objective has to be displaceable at right angles to a film carrier in order to adjust a sharp focus.
The objective holders or optical devices of the afore-described type are generally known in the art. The construction of such holders, however, has to meet various contradicting requirements which are frequently difficult to meet. For instance to achieve a uniformly sharp projected image it is necessary that the objective be exactly displaceable at right angles to the film. Also, for the sake of a good quality of the projected image the objective holder is to be designed without any play. Inasmuch as optical devices in many cases are subject to vibrations, the objective holder has to be designed such as to prevent any unintentional displacement of the objective which would require a repeated manual readjustment of the sharp focus. In the case of microfiche readers it is frequently also desired that the objective be changeable in a simple and fast manner for another one. Furthermore, since optical apparatuses are mostly mass produced, it is of importance that in order to achieve a low price the objective holder be also inexpensive in manufacture.
Prior art optical devices only partially meet the above-mentioned requirements. For example, a conventional objective holder employs a tubular piece having an inner thread in which the objective which is provided with an outer thread is screwed. The focussing is made exactly as in a photographic camera by turning the objective in the tubular piece.
Such a known objective holder, however, requires high manufacturing tolerances and is therefore expensive to manufacture. For instance, in microfiche film readers in many cases only a small marginal area of the objective is used for the transmission of the light beam. Furthermore, the objectives used in such mass produced optical devices have not for economic reasons the same optical quality over their entire lens surface. It may happen, therefore, that in an objective which is focussed by turning, the projected light beam passes through such an area of the objective surface in which the optical quality is relatively poor.
Also known is an objective holder which axially displaces the objective lens in order to adjust the sharp focus. This arrangement has the advantage that in the case of an assymetric transmission of the light beam it is possible during the installation of the objective into the holder to select such a position or range on the objective surface which ensures the best quality of the projection. This selected range or position is not changed during the axial displacement of the objective. Conventionally, the objective holders of this kind are assembled of two sleeves of which the inner sleeve supports the objective and is provided with a groove cooperating with an excenter so that by operating the excenter by means of an adjustment shaft the inner sleeve is axially displaced. This known construction also requires an extremely high manufacturing accuracy and therefore is also expensive. Any play between the two sleeves produces optical deterioration because due to the one-sided engagement of the excenter the inner sleeve has a tendency to tilt relative to the outer sleeve.